The present application relates to hydraulic systems, and more particularly to a hydraulic lock cylinder apparatus in hydrostatic circuits in a drilling rig.
When using hydraulic cylinders subject to suspended loads, a counter-balance valve retarding hydraulic fluid flow typically prevents the load from drifting or running away when being lowered. When extending, a bypassing check valve permits reverse flow around the counter-balance valve to allow the cylinder to extend.
When used in hydrostatic circuits, the counterbalance valve generates heat that is difficult to dissipate, and the counterbalance valve also requires external energy to operate. To lock cylinders in place on mobile equipment with a suspended load, a manual or counterbalance lock valve positioned between the cylinder and a directional valve typically prevents hydraulic leakage from the cylinder.
A prior art configuration using a hydraulic spool valve for directional control and a counterbalance valve to prevent runaway is shown in FIG. 2A-D. In FIG. 2A the hydraulic cylinder 105 supports a big heavy load 110, which can induce considerable pressure in the hydraulic cylinder 105. The counterbalance valve 115 is a special type of pilot controlled check valve used to control flow, and the hydraulic spool valve 120 functions as directional control for the fluid flow. In FIG. 2B, to lower the load 110, the counterbalance valve must be pressurized by the hydraulic fluid to open the counterbalance valve 115 so hydraulic fluid can flow from the hydraulic cylinder 105 and lower the cylinder 105. The pressurized fluid causes wasted energy.
Another difficulty in prior art systems, as shown in FIG. 2C, is handling differential cylinder flow. There is a volume difference in the hydraulic cylinder 105 due to space taken up by the cylinder rod 108. The volume in the rod section 109 is less than in the cap end 107. A smaller volume of hydraulic fluid exits the cylinder 105 than enters it when extending. A larger volume exits the cylinder 105 than enters it when retracting. In closed circuits, an auxiliary pump 125 is used to remove or add fluid to the system to compensate.
FIG. 2D shows a configuration for extending a load in a closed circuit system using a hydrostatic pump (over center). When using a hydrostatic pump (over center) 130, the extra flow required in the large cap end 107 must be replenished. The auxiliary pump 125 pumps fluid back into the system to make up for the differential cylinder flow, with the counterbalance valve 115 closing if pressure drops to prevent a runaway cylinder. The sum of the hydraulic fluid in and out of the main pump 130 must be equal
FIG. 2E shows a configuration for retracting a load in a closed circuit system using a hydrostatic pump (over center). When using a hydrostatic pump (over center) 130, the excess flow from the large cap end 107 must be exhausted at minimal pressure (e.g., charge pressure). The auxiliary pump 125 pumps fluid out the system to make up for the differential cylinder flow, with the counterbalance valve 115 closing if loss of pressure occurs to prevent a runaway cylinder. A second counterbalance valve 135 is required to help control flow and stop backflow in the event of loss of pressure in an exhaust circuit, as the main pump 130 also pumps fluid through the system.
In summary, the nature of the hydrostatic circuit in normal operation provides the necessary braking action to prevent the cylinder from running away. During normal operation of a hydrostatic circuit, a charge pressure is present in the main working lines. However, failure of a main fluid conductor or damage to the pump can cause a loss of charge pressure, which can result in an uncontrolled retraction and a run away cylinder. The loss of charge pressure when heavy loads are present can result in catastrophic damage.